Over the past few days I have been pulling my hair out trying to figure out why SharePoint Designer suddenly stopped working for creating workflows. This issue started occurring on both my own SharePoint site (P1 Plan) and all clients (Ranging between E1, E2, and E3). When creating a new list workflow, reusable workflow, or site workflow I would see the dialog box and give the workflow a name, but after clicking OK the dialog closed and nothing happened. The Problem You can’t create workflows using SharePoint Designer on your SharePoint Online / Office 365 SharePoint site. When you attempt to create a workflow, you complete the dialog for Name and Description, then it closes. No workflow is created, and you are not presented with the workflow design interface. Why it started happening The issue started happening due to the recent upgrades to SharePoint Online. Microsoft have been busy upgrading the back end of SharePoint online in preparation for the new Office 365 Plans such as Offic3 365 Small Business, which launched in the last day or two. Existing using of the ‘Classic’ P and E plans have started to be upgraded, in terms of the backend, but not everybody can fully upgrade to the new SharePoint 2013 front end (To check if you can upgrade yet try visiting: https://yoursitename.sharepoint.com/_layouts/siteupgrade.aspx). I believe you should be notified when the option to upgrade is available, but it could be worth checking back every once in a while just in case it becomes available to you. With this backend change you should switch to using SharePoint Designer 2013 (Download here) to manage your site correctly. Microsoft recommend you uninstall SharePoint Designer 2010 first, but you can technically install them both side by side. How to check if your backend has been upgraded, indicating you should update to SharePoint Designer 2013 To check is simple, connect your site in SharePoint designer, in the information panel to the right you will see various information such as the name of the site, description, web address, and SharePoint version. If the SharePoint version is 15 then you should switch to SharePoint Designer 2013. At last check mine read: SharePoint Version: 4 (15.0.0.4454) If after switching to SharePoint Designer 2013 you still can’t create workflows… If the problem is still occurring it is likely that the site has previously been edited using SharePoint Designer 2010. Follow these steps to resolve the problem and gain the ability to create workflows again: Close any open instances of SharePoint Designer Navigate to C:\users\yourusername\AppData\Local\Microsoft\WebsiteCache Delete the folders that relate to the site you are having the issue with, they will be named as “yoursitenamesharepointcom” Navigate to C:\users\yourusername\AppData\Roaming\Microsoft\SharePoint Designer\ProxyAssemblyCache Delete any folders in this location This solved my issue, after completing the above I was able to run up SharePoint Designer 2013 and creation of workflows is now working again. This is covered in the following Microsoft KB Article, but it was difficult to find, despite seeing others online having the same issues. I’ve seen responses online suggesting creating a whole new site in the Site Collection, for me this was not an option. This is much more convenient and worked right off the bat for me. Hope this post helps somebody find the answer they are looking for as I wasted at least two evenings trying to track down the issue.